I’m recently thinking about creating enamel pins so I can make some money and save up, but since I want to design the pin with anime’s such as Toilet Bound Hanako kun and Cardcaptor Sakura, I am afraid that I will get sued for making fan products.
Do we really need the copyright owner to approve if it is just a fan product?
For legal questions such as these, it is always good to contact a copyright lawyer in your country. Laws regarding fair use, copyrights and the likes differ per country, and a copyright lawyer should be more than capable of answering your questions. And this answer should not be seen as legal advice.
Beyond that, yes, you'd likely need a license. And should contact the original IP holder (or their legal departments) to obtain it see also Who do I contact regarding rights licenses of specific manga/anime characters?
To quote sharur's answer on Law.SE addressing the creation of fan-made merchandise of the game Overwatch.
... This is exactly the situation that IP law (e.g. copyright and trademark) was created to address. Blizzard created the game and so they have rights to control and benefit from derivatives there of. There are some exceptions, but prints, buttons, and keychains are not likely to meet the requirements for those.
Additionally, you can also take a look at Printing copyrighted logos on t-shirts/jackets/apparel without permission: in which situations is it legal? which covers another angle on a similar idea.
Legally, you will almost definitely need a license. In fact, pretty much all fan merch (including fanart) is technically illegal or at least in a legal gray area - there is little preventing companies with copyright ownership from striking down on such things, other than bad publicity.
That said, there is a general unspoken agreement, especially for Japanese anime and manga, that companies will turn a blind eye to small-scale operations* (like doujinshi, or some con-goer selling a few dozen postcards of fanart), as those will mostly help spread word-of-mouth and generally cannot serve to meaningfully replace official tat.
Problems occur with:
1 should be obvious - large scale operations, particularly of merch (less so with doujinshi or fanfiction, insofar as they can be "large-scale"), may significantly threaten the profits of the copyright holder by replacing their own merch of the same type.
2, in addition to more flagrant infringement, has the possibility of fooling people into thinking that they are official.
3 is a bit complicated - if the copyright holder gives explicit permission without a license, even to a pure-intentioned fan, apparently it may weaken their legal position when an actually problematic copyright infringement issue occurs. Thus, upon such inquiries, companies are often forced to make a public statement that selling fan creations are not allowed - which has caused cancellations of multiple single-work conventions.
In short, technically you do need a license (and I don't expect Kodansha or such to give out licenses to individual fans), but practically companies will typically not bother fans selling fan art pins or such merch in a small scale - although as with all unspoken rules, YMMV.
However, if you plan on using official art or sell on an industrial level, you may find yourself in some trouble.
*As an aside, this legal limbo is part of why you will often find unscrupulous merchants stealing fanart and slapping them on their merch to brazenly sell online or at cons - their dubious legal position makes artists hesitant to seek recourse.
